A substrate (SOI substrate) having an SOI (Silicon On Insulator) structure is known as a substrate having a single-crystal Si layer on an insulating layer. A device using this SOI substrate has many advantages that cannot be achieved by ordinary Si substrates. Examples of the advantages are as follows.                (1) The integration degree can be increased because dielectric isolation is easy.        (2) The radiation resistance can be increased.        (3) The operating speed of the device can be increased because the stray capacitance is small.        (4) No well step is necessary.        (5) Latch-up can be prevented        (6) A complete depletion type field effect transistor can be formed by thin film formation.        
Since an SOI structure has the above various advantages, researches have been made on its formation method for several decades.
As a method, an SOI structure is formed by bonding a single-crystal Si substrate to another thermally oxidized single-crystal Si substrate by annealing or an adhesive. In this method, an active layer for forming a device must be uniformly thin. More specifically, a single-crystal Si substrate having a thickness of several hundred micron must be thinned down to the micron order or less.
To thin the substrate, polishing or selective etching can be used.
A single-crystal Si substrate can hardly be uniformly thinned by polishing. Especially, in thinning to the submicron order, the variation range is several ten %. As the wafer size becomes large, this difficulty becomes more pronounced.
The present applicant has disclosed a new SOI technique in Japanese Patent Laid-Open No. 5-21338. In this technique, a first substrate obtained by forming a porous layer on a single-crystal Si substrate and a non-porous single-crystal layer on its surface is bonded to a second substrate via an insulating layer. After this, the bonded substrate stack is separated into two substrates at the porous layer, thereby transferring the non-porous single-crystal layer to the second substrate. This technique is advantageous because the film thickness uniformity of the SOI layer is good, the crystal defect density in the SOI layer can be decreased, the surface planarity of the SOI layer is good, no expensive manufacturing apparatus with special specifications is required, and SOI substrates having about several hundred-Å to 10-μm thick SOI films can be manufactured by a single manufacturing apparatus.
To separate the bonded first and second substrates into two substrates without breaking the first and second substrates, the following methods are available: the two substrates are pulled in opposite directions while applying a force in a direction perpendicular to the bonding interface; a shearing force is applied parallel to the bonding interface (for example, the two substrates are moved in opposite directions in a plane parallel to the bonding interface, or the two substrates are rotated in opposite directions while applying a force in the circumferential direction); pressure is applied in a direction perpendicular to the bonding interface; a wave energy such as an ultrasonic wave is applied to the separation region; a peeling member (e.g., a sharp blade such as a knife) is inserted into the separation region parallel to the bonding interface from the side surface side of the bonded substrate stack; the expansion energy of a substance filling the pores of the porous layer functioning as the separation region is used; the porous layer functioning as the separation region is thermally oxidized from the side surface of the bonded substrate stack to expand the volume of the porous layer and separate the substrates; and the porous layer functioning as the separation region is selectively etched from the side surface of the bonded substrate stack to separate the substrates.
As a method of separating a bonded substrate stack, the present applicant disclosed an epoch-making technique in Japanese Patent Laid-Open No. 11-45840 (Japanese Patent No. 2,877,800). In the separating method described in Japanese Patent Laid-Open No. 11-45840, a bonded substrate stack having a porous layer or ion implantation layer serving as a separation layer is separated into two substrates by injecting a fluid into the side surface of the bonded substrate.
More specifically, in the separating method described in Japanese Patent Laid-Open No. 11-45840, the bonded substrate stack is held by a pair of holders (substrate holding portions) having a smaller size than that of the bonded substrate stack. A fluid is injected into the side surface of the bonded substrate stack while rotating it, thereby separating the bonded substrate stack into two substrates at the porous layer.
In holding the bonded substrate stack by the pair of holders, which have a size smaller than that of the bonded substrate stack, distortion may occur at the peripheral portion due to its own weight or the like, or the bonded substrate stack may be held with some distortion which has occurred in the manufacturing process remaining without being corrected. Assume that distortion exists in the bonded substrate stack held by holders. In this case, when a fluid is to be injected throughout the perimeter of the bonded substrate stack while rotating it, the fluid may be injected to a position deviated from an appropriate position (e.g., a bonding interface or separation layer).
For the sake of easy understanding, an extreme case will be described. For example, if the bonded substrate stack is largely distorted, a fluid injected from a nozzle, the position of which has been adjusted on the assumption that the bonded substrate stack is not distorted, is not injected to the separation layer or bonding interface of the bonded substrate stack. When this occurs, separation processing may not progress at all. This will be described with reference to FIG. 13. FIG. 13 shows a state wherein a bonded substrate stack 50 which is largely distorted is separated into two substrates at a separation layer 12. In the example shown in FIG. 13, the position of the actual porous separation layer 12 largely deviates from that of a porous layer without any distortion (the ideal position of the porous layer). In this case, the position of a nozzle 120 is adjusted on the assumption that the porous layer 12 is arranged at the ideal position. When separation is performed in this state, a fluid from the nozzle 120 is not injected to the separation layer 12, thereby preventing smooth separation. If large distortion exists at the perimeter of the bonded substrate stack 50, separation does not progress speedily. This may result in unsatisfactory separation.
The probability of the above-described unsatisfactory separation would increasingly become higher due to increase in size of a bonded substrate stack along with increase in size of a required SOI substrate.
In addition, in holding a bonded substrate stack by holders, some foreign substance may enter between the bonded substrate stack and holders. In this case as well, a fluid from a nozzle may be injected to a position deviated from the porous layer or bonding interface of the bonded substrate stack.